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Author Topic: Real Aircraft question  (Read 11721 times)

Offline Chris McMillin

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Re: Real Aircraft question
« Reply #50 on: November 06, 2012, 04:38:00 PM »
Balderdash.

Said by one of those protected by the crew's being blamed.



Offline Chris McMillin

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Re: Real Aircraft question
« Reply #51 on: November 06, 2012, 04:53:50 PM »
Chris, I was just repeating what was told to me 30 yrs ago, but it is plausable. I realize the truth is there somewhere. The Flight Data recorders of the age were lacking in data collection compared to today. One of the mods I've done over the years is adding sensors to the MD-80 series to bring them into compliance. Today's FAA is staffed with alot of former Airline Pilots who bump heads with the NTSB over flight safety data and who controls it. 
The Airline I currently work for overhauled FedEx 727's in the mid/late 90's, so I probably worked on that airframe. I know I came across some ex-EAL "Shorties" in FedEx livery on that line. Geesh, we (EAL) got rid of them in 1984, they were in rough shape with high cycles back then.

You may have, Doug! EAL had a bunch of DC-9-50's that TWA leased from Guiness-Peat or someone in the late 90's, they were a mess too.

Offline Howard Rush

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Re: Real Aircraft question
« Reply #52 on: November 06, 2012, 05:43:04 PM »
Said by one of those protected by the crew's being blamed.

I didn't participate in the investigation, but I heard about it from those who did.  It was pretty much a slam dunk, even with the voice recorder erasure.
The Jive Combat Team
Making combat and stunt great again

Offline Chris McMillin

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Re: Real Aircraft question
« Reply #53 on: November 06, 2012, 06:08:21 PM »
Howard,
Please drop it, it's personal for me.
Chris...

Offline Ted Fancher

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Re: Real Aircraft question
« Reply #54 on: November 06, 2012, 06:46:57 PM »
As noted, you could probably Slow Roll a 727, but extended inverted flight would be a computer-generated image by Hollywood.  (Factual answers would come from Fancher, Fitzgerald, McMillin, Gleason, et al.)


Not a slow roll, Mike but, as Tex has shown dramatically, a barrel roll done properly is a one G maneuver all the way around and so the relationship of the CG, and the required download on the tail never changes.

Older Boeing airplanes require a significant download on the tail to stabilize their more forward Center of Gravity.   The slower they go (for a given flap setting) the more "nose up" trim has to be applied...in this case by lowering the leading edge of the horizontal stabilizer and producing downward (negative) lift of the tail to hold the nose up.

As I understand it, later model Airbuses are design to fly with further aft trim requiring little or (perhaps) no download on the tail in an ideal state (pax and fuel in the "ideal spread" fore and aft).  It is probably conceivable that such a ship could be flown in one g inverted flight although if the tail could drive the wing to a positive angle of attack while inverted (consider the cambered airfoil's need to be pitched "up" relative to the airflow to produce lift when inverted).

It is highly unlikely (although Mr. Rush will correct me if I'm too far out on a limb) that any Boeing with which I'm familiar could be driven to a high enough angle of attach when inverted to maintain level flight.

Whoever mentioned fuel feed is pretty much on the money.  The fuel pumps and pickups are located in the lower portions of the tanks and sustained inverted flight...if aerodynamically possible...would be a short lived exercise.

Ted

Offline Ted Fancher

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Re: Real Aircraft question
« Reply #55 on: November 06, 2012, 06:49:45 PM »
Fuel and oil systems and flight control systems aside, sustained inverted is a negative G maneuver, whereas a barrel roll maintains positive G's. I don't know how many negative G's the airframe is designed for, but I suspect inverted flight was not within the design envelope!

Bill,

Inverted flight is a one G maneuver and air carrier aircraft are certified to well beyond that.  Minus three or so sounds sort of familiar but my books are all at work.  Shoot.  I might have "experienced" a couple of 2g (at a minimum) landings in my career!  One of which was in a 727.  By the way.  I've only seen a quick glance at that trailer once and my visual memory was more like a DC-9 or MD-80.  Not at all sure.

Ted

Offline Ted Fancher

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Re: Real Aircraft question
« Reply #56 on: November 06, 2012, 06:59:02 PM »
Sounds like a big bunch of  Negative G..  No way Hosea  :)

I'd agree 100% Dan.  Again, it "may" be possible to stop a barrel roll while inverted and apply enough down elevator to keep the nose from dropping at 1g but there just doesn't seem to me to be anyway to pull out of a steep dive inverted without exceeding: A. the control forces (lift) necessary to do so or, B the negative g limits of the structure if there was enough control authority.

Again, I find it unlikely that sustained inverted flight even with the power sticking around somehow would be possible given the range of trim adjustments available in the stabilizer and the normal CG range against which the tail would need to work to hold the nose up.

Ted

p.s.  My guess is that the engines would fail due to lack of fuel long before lubrication became an issue.  Not so, necessarily with a recip but jets aren't nearly as susceptible to rapid disassembly due to reduced levels of lubrication.


Offline Mike Keville

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Re: Real Aircraft question
« Reply #57 on: November 06, 2012, 07:05:30 PM »
Whoops...semantics there, Ted.  I meant to say 'barrel' roll, not 'slow' roll.

That said, I still wish you'd consider writing a book about your experiences....everything from coastal runs with the CV-440 to the Dash-400 finale.  So many of us ground-bound 'wannabes' enjoy those tales.  Case-in-point being Len Morgan's "View From The Cockpit" (Sunflower University Press, June 1985).

We'd love to hear more, if only you would.

Cheers....
FORMER member, "Academy of Multi-rotors & ARFs".

Offline Ted Fancher

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Re: Real Aircraft question
« Reply #58 on: November 06, 2012, 07:09:19 PM »
Whoops...semantics there, Ted.  I meant to say 'barrel' roll, not 'slow' roll.

That said, I still wish you'd consider writing a book about your experiences....everything from coastal runs with the CV-440 to the Dash-400 finale.  So many of us ground-bound 'wannabes' enjoy those tales.  Case-in-point being Len Morgan's "View From The Cockpit" (Sunflower University Press, June 1985).

We'd love to hear more, if only you would.

Cheers....

Thanks for the suggestion, MIke, but.... I'm afraid you'd find my 38 years at the helm (or close to it) pretty boring.  Which is exactly the way we wanted it.  Thanks to Boeing, Douglas, some great flight training and excellent maintenance throughout my career.

Good to hear from you, my friend.

Ted

Offline Mike Keville

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Re: Real Aircraft question
« Reply #59 on: November 06, 2012, 07:16:01 PM »
....I'm afraid you'd find my 38 years at the helm (or close to it) pretty boring.  Which is exactly the way we wanted it....  Ted

<chuckle>...Seems reasonable, Ted.  'Boring' is good in that business (said as a frequent PAX!)

See you in Tucson next March?

FORMER member, "Academy of Multi-rotors & ARFs".

Offline Bob Whitely

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Re: Real Aircraft question
« Reply #60 on: November 06, 2012, 08:20:06 PM »
Some of you guys really need to pay attention when things are
explained by those that know what they are speaking about. Not one
of you is on a level field with either Chris or Ted so you may want
to re-evalute how you phrase your comments as they indicate the
lack of knowledge and expertise of the subject under discussion. I have
flown with both of them and they are as good as it gets. If you want
to make flatout statements try to get them right or it makes you
look, well, you know, not really up to speed...I'm just sayin'.  RJ

Offline Mike Keville

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Re: Real Aircraft question
« Reply #61 on: November 06, 2012, 08:37:03 PM »
I think what he's trying to say is that you can believe whatever Ted and/or Chris tells you about flying high-performance passenger aircraft.

Everything else is B.S.
FORMER member, "Academy of Multi-rotors & ARFs".

Offline Steve Fitton

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Re: Real Aircraft question
« Reply #62 on: November 06, 2012, 09:41:05 PM »
Thanks for the suggestion, MIke, but.... I'm afraid you'd find my 38 years at the helm (or close to it) pretty boring.  Which is exactly the way we wanted it.  Thanks to Boeing, Douglas, some great flight training and excellent maintenance throughout my career.

Good to hear from you, my friend.

Ted

I think Mike's point was that you make the "boring" very entertaining. (Tales of the approach into Kai Tak come to mind)
Steve

Offline Ted Fancher

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Re: Real Aircraft question
« Reply #63 on: November 07, 2012, 01:38:51 PM »
I think Mike's point was that you make the "boring" very entertaining. (Tales of the approach into Kai Tak come to mind)

Ah, Yes.  Steve.  Kai Tak! 

All I can tell you about that approach is that you could tell which of the very pretty Chinese gals wore thongs because you could see them hanging on the clothes lines out their apartment windows to dry as you turned close final!

How's that for entertaining?  <= <= n1

Ted

p.s.  I might add, for the heck of it, that the dearth of accidents at Kai Tak--despite the infamous approach (google it for videos for a few hours of entertainment)--was a testament to the ability of stress to bring out the best in people.  Believe me, nobody ever flew that approach with their mind on something else.  Everyone in the cockpit paid attention and said something if something needed to be said.  Reminds me of how overwhelmingly good were the people I flew with for all those years.  Very proud of those associations.

Offline Ted Fancher

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Re: Real Aircraft question
« Reply #64 on: November 07, 2012, 02:54:37 PM »
I think what he's trying to say is that you can believe whatever Ted and/or Chris tells you about flying high-performance passenger aircraft.

Everything else is B.S.

Ah, Mike and Bob.  Not B.S., just discussion.  By definition, the guys on SH are fascinated by flying things and have one degree or another of knowledge about the subject.  Not all have the same degree but none of us have all the answers (as Mr. Rush is often pleased to point out).  A forum is for discussion out of which, from time to time, wisdom will surface and other times not.  I note that, generally, somebody will note that guys with a bit more experience might add more to the discussion which is a tacet acknowledgement that they're mostly aware that the discussion to date might not have provided all the answers.

I love it when these discussions take place and often sit back and see if somebody doesn't eventually hit the nail on the head.  There were a few near misses in this thread, each of which added a bit of value to the mix.

Ted

p.s. Here's a bit of observable info on the subject of air carrier "pitch trim settings"  which those of you living near airports can check out which will help explain the problem with Denzel's heroic efforts in the film.

If you can get to an observation spot near the end of a runway where jetliners are starting their takeoff roll take a close look at the horizontal stabilizers as they take the runway.  For the most part Boeing airplane will have the stabilizer set with the leading edge well below the trailing edge, sort of like giving up elevator to a flying tail on a combat ship.  (big jets trim pitch by changing the angle of incidence on the stab, not by using trim tabs on the elevators).

The exact angle at which the stabilizer is set is computed for each takeoff and, if correct, will result in an initial climb attitude that requires no elevator input and delivers an indicated airspeed of "V2+10 knots" (V2 being one of three speeds computed for each takeoff considering aircraft weight, density altitude, runway length, flap setting and so forth.  The other two speeds are V1 (the "go"/"no go" speed to either safely reject the takeoff on the remaining runway or, if above that speed, continue the takeoff following an engine failure and cross the far end of the runway at a minimum of 35 feet and meet all the angle of climb requirements for terrain, etc. while climbing out at V2+10K.  The third speed is Vr, the speed at which the flying pilot rotates the nose to break ground and establish the climb)  Sorry, TMI and not exactly pertinent to the discussion.

This significant Nose Up trim is required by the Boeing jets because they are designed so as to require significant download on the tail to maintain an "in trim" condition for any given speed and/or vertical flight path--i.e. climb/level flight/descent.  The download is required because the Center of Gravity is located forward of the Center of where the lift to support it is generated.  (By the way if this is starting to sound familiar it's part and parcel of the discussion of aft CGs with stunt ships that has been a subject of great interest and discussion in the last 15 or 20 years.)

You can see the effects of this CG/CL relationship when watching Boeing jets on final approach (ideally inside five miles or so from the end of the runway when they are stabilized at approach speed with landing flap settings).  Boeings will be in a noticeably nose high attitude and with high thrust settings in this configuration and the stab setting will be close to the maximum leading edge down (nose up trim) setting to maintain this attitude with no pilot elevator input.

To get a mental feel for what I'm talking about here, imagine adding more and more weight to the nose of your stunt ship.  At some point in time the CG will be so far ahead of where the lift to support it is generated that it will take all the up elevator available just to fly level and maneuvering will not be an available option.  The Boeing operates in that direction but, of course, not to that extreme.

By the way, if you look closely at the Boeings as they roll onto the runway you'll see the "track" in which the mechanism drives the leading edge of the stab up and down to adjust trim.  You'll easily see that there is lots of "nose up" movement available but very little nose down (A great amount of "nose down" trim would be required to even pretend to fly inverted in the Boeing and is simply not available to the best of my guess!)  The overall design concept of the Boeing is to minimize download required in cruise with the flaps up and at high speed to minimize SFC (Specific Fuel Consumption) while sacrificing any such fuel efficiency during slow speed regimes.

(A caveat, I'm uncertain about the CG/CL relationships on late model Boeings like the 777 and 787.  They could well be more like the Airbusses which I'll touch on next)

Late model airbuses are substantially different in this regard.  A large part of their popularity with airlines is the design concept that more approximates the aft CG stunt ship.  Tail authority (and  computerized control inputs for stabilitly and maneuvering) that allow aft CGs which even further minimize the need for tail download which reduces SFC to an even greater degree (the single biggest selling factor to airlines is, understandably, how much gas the jets will burn for each butt in the seat mile).

Airbuses on short final will tend to have "flatter" body angles and not require as much thrust (as a for instance, a Boeing 727 with full flaps at approach speed required on the order of 80% of maximum N1 (forward rotor speed) just to maintain a three degree glide slope.  For another "recent stunt dialogue" comparison that high thrust is also a result of the great drag and negative pitching moment from the huge triple slotted flaps!  They're all airplanes...some's just bigger than others!)

If you watch the Airbuses taxi out you'll notice a distinctly different stab trim setting situation.  The stab angle of incidence (its angle with respect to the fuselage) will be much closer to zero/zero with respect to the wing (that relationship, wing angle of incidence in relation to stab angle of incidence is sometimes termed decalage although I understand that term is not universally embraced by engineers...not sure why). 

The bottom line of all this with respect to Denzel's wonderful adventure is that Boeings (and probably DC9s, although I've no personal experience with them) are extremely ill suited to negative g (inverted) flight as the combined elevator and trim forces necessary to maintain the necessary high angle of attack inverted are almost certainly not available.  The Airbus model might have a modestly better potential but I wouldn't want to be the test pilot.d\

Hope this didn't put you all to sleep.

Ted


Offline Steve Thomas

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Re: Real Aircraft question
« Reply #65 on: November 07, 2012, 04:21:28 PM »
Quote
Airbuses on short final will tend to have "flatter" body angles

Except, for some reason, the A330 (and I guess the A340). They fly down final significantly more nose-up than comparable Boeings - about 4 degrees NU vs 2.5. I guess it's to do with the relationship between the leading edge and trailing edge devices, but someone smarter than me would have to answer that one.  

A couple of my colleagues had a very nasty incident in an A330 where erroneous air data signals caused some dramatic pitch excursions, despite the fact that they disconnected the autopilot straight away (there's something to be said for old-tech, non-FBW aircraft...). I don't remember the exact numbers, but the thing was overstressed in both positive and negative G, and went to more than -1G.
« Last Edit: November 07, 2012, 05:01:29 PM by Steve Thomas »

Offline Ted Fancher

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Re: Real Aircraft question
« Reply #66 on: November 07, 2012, 06:51:51 PM »
Except, for some reason, the A330 (and I guess the A340). They fly down final significantly more nose-up than comparable Boeings - about 4 degrees NU vs 2.5. I guess it's to do with the relationship between the leading edge and trailing edge devices, but someone smarter than me would have to answer that one.  

A couple of my colleagues had a very nasty incident in an A330 where erroneous air data signals caused some dramatic pitch excursions, despite the fact that they disconnected the autopilot straight away (there's something to be said for old-tech, non-FBW aircraft...). I don't remember the exact numbers, but the thing was overstressed in both positive and negative G, and went to more than -1G.

That could well be, Steve.  The ones I watch out my den window (I'm just about abeam the outer marker at SFO) are mostly 319s and 320s.

Ted



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